Solution to optical constraint on microtruss processing

ABSTRACT

A system for fabricating a radiation-cured structure is provided. The system includes a radiation-sensitive material having a first refractive index; a mask formed from a mask material having a second refractive index; and a radiation source. The mask is disposed between the radiation source and the radiation-sensitive material, and has a plurality of substantially radiation transparent apertures. The radiation source is configured to generate radiation beams for at least one of initiating, polymerizing, and crosslinking the radiation-sensitive material. The system includes at least one of a) an at least one normalizing surface disposed between the radiation source and the mask, b) a refractive fluid having a third refractive index disposed between the radiation source and the mask, and c) the refractive fluid having the third refractive index disposed between the mask and the radiation-sensitive material. A method for fabricating the radiation-cured structure is also provided.

FIELD OF THE INVENTION

The present disclosure relates to a fabrication of radiation-curedstructures and, more particularly, to a method for fabricating aradiation-cured microtruss.

BACKGROUND OF THE INVENTION

Microtruss structures formed by photopolymerization have been describedby Jacobsen et al. in “Compression behavior of micro-scale trussstructures formed from self-propagating truss elements”, Acta Materialia55, (2007) 6724-6733, the entire disclosure of which is herebyincorporated herein by reference. One method and system of creatingpolymer materials with ordered microtruss structures is disclosed byJacobsen in U.S. Pat. No. 7,382,959, the entire disclosure of which ishereby incorporated herein by reference. Microtruss materials producedby the method and system are further disclosed by Jacobsen in U.S.patent application Ser. No. 11/801,908, the entire disclosure of whichis hereby incorporated herein by reference. A polymer material that isexposed to radiation and results in a self-focusing or self-trapping oflight by formation of truss elements is also described by Kewitsch etal. in U.S. Pat. No. 6,274,288, the entire disclosure of which is herebyincorporated herein by reference.

As shown in FIG. 1, the known systems for fabricating microtrussstructures may include at least one collimated light source 100 selectedto produce a collimated light beam 102; a reservoir 104 having aphotomonomer 106 adapted to be polymerized by the collimated light beam102; and a mask 108 having at least one aperture 110 and positionedbetween the at least one collimated light source 100 and the reservoir104. The collimated light source 100 is generally a mercury arc lampconfigured to produce collimated ultraviolet (UV) light beams at adesired angle. The mask it typically formed on a layer of quartz glass112. A light boundary 114 exists between the quartz glass 112 and theair, and the quartz glass 112 and the photomonomer 106, due to thedifferences in index of refraction between the respective media. The atleast one aperture 110 is adapted to guide a portion of the collimatedlight beam 102 into the photomonomer 106 to form the at least one trusselement 116 through a portion of a volume of the photomonomer 106.Multiple truss elements can be formed simultaneously from a singlecollimated light beam 102 that travels along a path from the lightsource 100, through the mask 108 and the quartz glass 112, and into thereservoir 104 of the photomonomer 106.

The formation of microtruss structures from the known methods, however,has been constrained by an optical phenomenon known as Snell's Law.Snell's Law states that the ratio of the sines of the angles ofincidence and refraction is equivalent to the opposite ratio of theindices of refraction. For example, Snell's Law can be represented asn ₁ sin θ₁ =n ₂ sin θ₂where n₁ and n₂ denote the first and second media and the angles ofincidence and refraction are measured with respect to the normal to theinterface between the media. A similar application of Snell's Law to athird and fourth media can be represented asn ₁ sin θ₁ =n ₂ sin θ₂ =n ₃ sin θ₃ =n ₄ sin θ₄

For a given set of indices where n₂>n₁, there is limiting angle θ₂corresponding to the physical limit for the incoming angle θ₁ of 90degrees (parallel to the interface.) For the case of light passing fromair into a quartz glass mask substrate, the indices of 1.0003 and 1.46respectively, the limiting angle in the glass substrate is 43 degrees asdetermined by solving for θ₂ in the equation n₁ sin θ₁=n₂ sin θ₂.

Under Snell's Law, the refracted angle (θ) is greater than the angle ofincidence (α), and the truss elements 116 having desirably largerefracted angles (θ) cannot be produced through conventional means wherethe boundary 114 between the air and the quartz glass 112 exists.

With renewed reference to FIG. 1, a practical example with an incidentangle (α) of about 68 degrees is shown. The refracted angle (θ) in thequartz glass 112 is about 40 degrees, or only 3 degrees less than thetheoretical limit of 43°. Since the faces of the quartz glass 112 areparallel in FIG. 1, the incident angle (α) as the collimated light beam102 exits the quartz glass 112 is also about 40 degrees. As thecollimated light beam 102 proceeds into the photomonomer 106 with anindex of about 1.51, the resulting refracted angle (θ) is about 38degrees

There is a continuing need for a system and method for fabricatingradiation-cured structures with truss elements disposed at anglesgreater than about 45°, and in particular at angles of greater thanabout 60°, with respect to normal to the refractive boundary surface.Desirably, the system and method enables production of large-angledradiation-cured structure features, including microtruss structures.

SUMMARY OF THE INVENTION

In concordance with the instant disclosure, a system and method forfabricating radiation-cured structures with truss elements disposed atangles greater than about 45°, and in particular at angles of greaterthan about 60°, with respect to normal to the refractive boundarysurface, which enables production of large-angled radiation-curedstructure features, including microtruss structures, is surprisinglydiscovered.

In a first embodiment, a system for fabricating a radiation-curedstructure includes a radiation-sensitive material having a firstrefractive index, a mask formed from a mask material having a secondrefractive index, and a radiation source. The mask is disposed betweenthe radiation-sensitive material and the radiation source. The mask hasa plurality of substantially radiation transparent apertures formedtherein. The radiation source is configured to generate radiation beamsfor at least one of initiating, polymerizing, crosslinking, anddissociating the radiation-sensitive material. The system includes atleast one of a) an at least one normalizing surface disposed between theradiation source and the mask, b) a refractive fluid having a thirdrefractive index disposed between the radiation source and the mask, andc), the refractive fluid having the third refractive index disposedbetween the mask and the radiation-sensitive material.

In another embodiment, the system includes the radiation-sensitivematerial having the first refractive index, the radiation source, andthe mask having the second refractive index. The refractive fluid havingthe third refractive index is disposed between the mask and theradiation-sensitive material. A prism having at least one normalizingsurface is also disposed between the radiation source and the mask.

In a further embodiment, a method for fabricating a radiation-curedstructure, includes the steps of: providing a radiation-sensitivematerial having a first refractive index, a mask having a plurality ofsubstantially radiation transparent apertures formed therein, the maskformed from a mask material having a second refractive index, and aradiation source configured to generate radiation beams for at least oneof initiating, polymerizing, crosslinking, and dissociating theradiation-sensitive material; placing the mask between theradiation-sensitive material and the radiation source; at least one ofa) disposing an at least one normalizing surface between the radiationsource and the mask, b) disposing a refractive fluid having a thirdrefractive index between the radiation source and the mask, and c)disposing the refractive fluid having the third refractive index betweenthe mask and the radiation-sensitive material; and exposing theradiation-sensitive materials to a plurality of radiation beams throughthe radiation transparent apertures in the mask. The radiation-curedstructure is thereby formed.

DRAWINGS

The above, as well as other advantages of the present disclosure, willbecome readily apparent to those skilled in the art from the followingdetailed description, particularly when considered in the light of thedrawings described herein.

FIG. 1 is a schematic side sectional view and top plan illustrationshowing a prior art system for forming radiation-cured structures;

FIG. 2 is a schematic side sectional view of a system according to afurther embodiment of the present disclosure, showing a prism disposedadjacent a mask;

FIG. 3 is a schematic side sectional view of a system according toanother embodiment of the present disclosure, showing a mask and aradiation-sensitive material immersed in a reservoir of a refractivefluid;

FIG. 4 is a schematic side sectional view of a system having a facetedmask disposed adjacent a radiation-sensitive material;

FIG. 5 is a schematic side sectional view of a system having anintervening refractive fluid between a mask and a radiation-sensitivematerial;

FIG. 6 is a schematic side sectional view of an alternative systemaccording to the present disclosure, having the prism as shown in FIG. 2and the intervening refractive fluid as shown in FIG. 5;

FIG. 7 is a schematic side sectional view of a system having the prismof FIG. 2 and a reservoir of the refractive fluid as shown in FIG. 3;

FIG. 8 is a schematic illustration of a system having the faceted maskas shown in FIG. 4 and the intervening refractive fluid as shown in FIG.5; and

FIG. 9 is a schematic illustration of a system as described in anexemplary embodiment according to the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description and appended drawings describe andillustrate various embodiments of the invention. The description anddrawings serve to enable one skilled in the art to make and use theinvention, and are not intended to limit the scope of the invention inany manner. In respect of the methods disclosed, the steps presented areexemplary in nature, and thus, are not necessary or critical.

As shown in FIGS. 2-9, the present disclosure includes a system 200 forfabricating a radiation-cured structure 201, such as a microtrussstructure having a plurality of elements 202, for example. Exemplaryarchitectures of the microtruss structure are described by Jacobsen inU.S. Pat. No. 7,382,959 and U.S. patent application Ser. No. 11/801,908.Although the system 200 may be described herein with respect to theformation of microtruss structures, one of ordinary skill in the artshould appreciate that other radiation-cured structures 201 may also befabricated within the scope of the present disclosure.

The system 200 includes a radiation-sensitive material 203 having afirst refractive index, a mask 204 formed from a mask material 206having a second refractive index, and a radiation source 208 configuredto generate a plurality of radiation beams 210 for at least one ofinitiating, polymerizing, crosslinking, and dissociating theradiation-sensitive material 203. The radiation-cured structure 201 ofthe present disclosure may be formed from the radiation sensitivematerial 203 as described, for example, in Assignee's co-pending U.S.patent application Ser. No. 12/339,308, the entire disclosure of whichis hereby incorporated herein by reference. The mask 204 is disposedadjacent the radiation-sensitive material 203 and has a plurality ofsubstantially radiation transparent apertures 212 formed therein. Theradiation source 208 is disposed adjacent the mask 204 opposite theradiation-sensitive material 203.

At least one refractive boundary 211 may exist in the system 200. Therefractive boundary 211 is defined herein as any interface between twomedia in the system 200 having different indices of refraction. Forexample, the system 200 may have the refractive boundary 211 at theinterface of the mask 204 with the radiation-sensitive material 203, atthe interface of a prism 226 (shown in FIGS. 2, 6, 7, and 9) with air,at the interface of the prism 226 with the mask 204, and at theinterface of the mask 204 with the radiation source 208, where theradiation source 208 abuts the mask 204 (not shown). It should beunderstood that the radiation beams 210 may bend as they cross therefractive boundaries 211 of the system 200 in a phenomenon known asrefraction.

Illustratively, the radiation beams 210 may approach the refractiveboundary 211 at an angle of incidence (α) relative to normal to a planeformed by the refractive boundary 211. The radiation beams 210, uponcrossing the refractive boundary 211, are bent according to Snell's Law,as described hereinabove, and have a refracted angle (θ) relative tonormal to the plane formed by the refractive boundary 211. When theindex of refraction of the first medium is less than the index for thesecond medium, the refracted angle (θ) is less than the angle ofincidence (α) and ultimately defines the angle of the radiation-curedelements 202 and the geometry of the resulting radiation-cured structure201 after the net result of bending at each refractive boundary 211.

The radiation-sensitive material 203 may be supported by a processingsubstrate 214, for example. The processing substrate 214 may be disposedatop a stationary base plate 216 during the fabrication process. Thesubstrate 214 may further be provided with a coating or surfacetreatment (not shown) for bonding and debonding from the radiation-curedstructure 201 after the fabrication thereof from the radiation-sensitivematerial 203. A backside of the substrate 214 typically disposed on thestationary base plate 216 during fabrication of the radiation-curedstructure 201 may also have a coating to militate against an undesiredcontamination of the substrate 214. The stationary base plate 216 mayinclude a porous vacuum chuck having a pressure-facilitated release, forexample, for selectively holding the substrate 214 in place during thefabrication process. A skilled artisan may select suitable surfacetreatments, including coatings, as desired.

Alternatively, it should be appreciated that the radiation-sensitivematerial 203 may be provided as a free standing film with no substrate214 in lieu of the step of providing the processing substrate 214 asdescribed hereinabove.

The radiation-sensitive material 203 includes at least one of aradiation-curable material and a radiation-dissociable material. Theterm “radiation-curable material” is defined herein as any material thatis at least one of initiated, polymerized, and crosslinked by exposureto radiation. It should be appreciated that an increase in temperaturemay also be employed to at least partially complete polymerization orcrosslinking of the radiation-curable materials following an initiationby the exposure to radiation. The term “radiation-dissociable material”is defined herein as any material that exhibits at least one of ascissioning of the polymer backbone and a decrosslinking by exposure toradiation. As a nonlimiting example, the radiation-dissociable materialmay be made solvent-soluble by a sufficient breakage of crosslinksand/or scissioning of the polymer backbone of the radiation-dissociablematerial.

As nonlimiting examples, the radiation-curable materials may include oneof a liquid photomonomer and a substantially solid radiation-curablepolymer. The radiation-sensitive material 203 may be a liquidphotomonomer as described by Jacobsen in U.S. Pat. No. 7,382,959 andU.S. application Ser. No. 11/801,908. Nonlimiting examples of suitablephotomonomers include monomers that polymerize via free-radicalpolymerization when exposed to UV radiation (wavelength between about250 nm and about 400 nm). The photomonomer may include any suitablefree-radical photomonomer material such as urethanes (polyurethanes),acrylates, methacrylates, and cationic polymers such as photo-curedepoxies. Suitable liquid photomonomers may exhibit a shift in index ofrefraction upon photopolymerization, for example, to provideself-propagating waveguides. Other photomonomers may also be employed,as desired.

Suitable substantially solid radiation-curable polymers may includenegative resist polymers. Negative resist polymers go through aphotoinitiation process that leads to a curing of the negative resistpolymer by polymerization or polycondensation, for example. Where thepolymerization or polycondensation reaction occurs at substantially thesame time, the process is referred to as “photocured”. Where only thereaction species are generated by the photoinitiation process and asubsequent step such a heating is required to generate thepolymerization or polycondensation, the process is be referred to as“photoinitiated”. It should be appreciated that even though a post-cureheat treatment may be necessary to finalize the polymerization step,substantially stable radiation-cured features in the negativephotoresist polymer may also be created during the initial radiationexposure. The substantially solid radiation-curable polymers can gothrough just the initiation process and, due to inherent stability andthe limited diffusion rate of the chemical species within the solidradiation-curable polymers, the curing process may also be performedmuch later without significant feature degradation. It should beappreciated that most photoinitiated polymers begin the curing processat the inception of the initiation process, but the kinetics of thereaction at the exposure temperature are so slow that little, if any,polymerization or polycondensation may take place prior to heating thenegative resist polymer to a desired curing temperature.

One particular negative resist polymer is the epoxy-based SU-8 2000™negative resist polymer, commercially available from MicrochemCorporation in Newton, Mass. The SU-8 2000™ negative resist polymer iscurable by UV radiation. It should be appreciated that othersubstantially solid radiation-curable polymers may be employed. Forexample, similar to the photomonomers described above, theradiation-curable polymer selected may be cured with radiation ofwavelengths other than UV radiation, as desired. The radiation-curablepolymer may also be selected to have a slower cure rate than the liquidphotomonomer, for example, to militate against features from appearingin the slower curing layer with exposure of the faster curing layer to aradiation source.

As a nonlimiting example, the radiation-dissociable materials mayinclude positive resist polymers. Positive resist polymers begin ascrosslinked polymers but may contain photoinitiators that, when exposedto a particular radiation, generate chemical species which dissociatethe polymer by at least one of breaking the crosslinks and scissioningthe polymer backbone. The dissociation makes the positive resist polymersoluble in the regions which have been exposed to the radiation. Regionswhere the positive resist polymer remains cured are masked rather thanbeing exposed, as is the case with the negative resist polymersdescribed hereinabove. In certain embodiments, the positive resistpolymers are sensitive to radiation, e.g., ultraviolet radiation,without the need for photoinitiators. For example, the positive resistpolymer may itself be damaged by the radiation and the remainingscissioned chains become soluble in a solvent. Other types of positiveresist polymers may be employed, as desired.

The mask material 206 forming the mask 204 is a substantially radiationtransparent material, such as quartz glass, for example. The apertures212 may be holes or substantially radiation transparent apertures formedin an otherwise opaque, radiation-blocking coating disposed on thequartz glass, for example. In one illustrative embodiment, the mask 204has a plurality of apertures 212. As further nonlimiting examples, themask material 206 may include one of crown glass, Pyrex glass, and apolyethylene terephthalate, such as a Mylar® film. The mask 204 may belifted away after an exposure and cleaned for reuse. The apertures 212may have shapes that provide the radiation beams 210, and therebyradiation-cured elements 202, with desired cross-sectional shapes. Forexample, the apertures 212 may be substantially circular to result information of radiation-cured elements 202 with ellipticalcross-sectional shapes. A skilled artisan may select suitable maskmaterials, aperture sizes and shapes, and resulting structuralconfigurations, as desired.

The radiation source 208 generates electromagnetic radiation. Theradiation beams 210 employed to cure the radiation-sensitive material203 may be generated by a Mercury arc lamp providing ultraviolet (UV)radiation beams 210, for example. A skilled artisan understands thatradiation beams 210 of other wavelengths, such as infrared, visiblelight, and X-ray radiation, and from other sources, such as incandescentlights and lasers, may also be employed. It should be further understoodthat the radiation beams 210 may be collimated, partially collimated, ornon-collimated, as desired. It should be further understood that theradiation source 208 for the present system 200 may providesubstantially monochromatic radiation beams 210 that will remaincollimated after passing through normalizing surfaces such as prisms orfaceted masks (shown in FIGS. 2, 4, and 6 to 9).

With reference to FIG. 2, the system 200 may include an at least onenormalizing surface 224 disposed between the radiation source 208 andthe mask 204. The normalizing surface 224 is configured to allow theradiation beams 210 from the radiation source 208 to intersect thenormalizing surface 224 substantially at normal relative to a planedefined by the normalizing surface 224. The normalizing surface 224 maybe configured to allow the radiation beams 210 to intersect thenormalizing surface 224 at an angle relative to normal that is less thanan angle that the radiation beams 210 would intersect a surface of themask 204 absent the normalizing surface 224. As shown in FIG. 2, the atleast one normalizing surface 224 may be provided by a prism 226.Illustratively, the prism 226 is formed from a substantially radiationtransparent material, such as quartz glass. The prism 226 may be a prismhaving a geometrical triangular or rectangular pyramid shape, althoughoptical prisms of other shapes may also be employed. The rectangularpyramid shape may be particularly advantageous due to the ease ofmanufacture. Other suitable radiation transparent materials may also beemployed. It should be appreciated that the mask 204 may be integrallyformed with the prism 226 to militate against pockets of air formingtherebetween.

In a particular embodiment, the prism 226 may include four normalizingsurfaces 224 configured to allow the radiation beams 210 to intersectthe normalizing surfaces 224 at an angle substantially perpendicularthereto. It should be appreciated that any number of normalizingsurfaces 224 may be employed, as desired. Refraction of the radiationbeams 210 at the refractive boundary 211 between the radiation source208 and the mask material 206, and in particular between the air and themask material 206, may thereby be minimized.

As shown in FIG. 3, the system 200 of the present disclosure may includea refractive fluid 218. The refractive fluid 218 has a third refractiveindex. The third refractive index may be selected to minimize a changein refractive index at the refractive boundary 211, for example, betweenthe air and the mask 204 or between the mask 204 and theradiation-sensitive material 203. Desirably, the third refractive indexmay minimize a difference between the angle of incidence (α) and therefracted angle (θ) of the radiation beams 210 as they cross therefractive boundary 211. In a particular embodiment, the thirdrefractive index is selected to minimize the critical angle (θ_(c)) andfacilitate the fabrication of radiation-cured elements 202 withdesirably large-angled features with respect to normal.

As nonlimiting examples, the third refractive index (refractive fluid218) may be at least one of intermediate the first refractive index(radiation-sensitive material 203) and the second refractive index (mask204), substantially the same as the first refractive index, andintermediate the refractive index of air and the second refractiveindex. For desirable tolerance control, the third refractive index mayparticularly be substantially the same as the first refractive indexwhen the radiation-sensitive material 203 is drawn against a vacuumchuck. The refractive fluid 218 may be selected from any substantiallyradiation transparent fluid having the desired third refractive index.The refractive fluid 218 also is selected to not significantly dissolveor degrade the radiation-sensitive material 203 and the mask 204 duringthe fabrication of the radiation-cured structure 201. For example, therefractive fluid 218 may include one of water, an aqueous sugarsolution, acetone, and mineral oil. One of ordinary skill in the art mayselect other suitable refractive fluids 218, as desired.

In one embodiment, the refractive fluid 218 may be disposed between themask 204 and the radiation source 208. As illustratively shown in FIG.3, the refractive fluid 218 may be disposed between the mask 204 and theradiation source 208 by at least partially immersing the mask 204 andthe radiation source 208 in a reservoir 220 of the refractive fluid 218.It has been recognized that the thin layer of air disposed at therefractive boundary 211 between the mask 204 and the radiation-sensitivematerial 203 may also be sufficient to limit the critical angle (θc),and thus limit the angles of features in the resulting radiation-curedstructure 201. The refractive fluid 218 desirably replaces any residualair that would otherwise be present between the radiation-sensitivematerial 203 and the mask 204.

Where the system 200 has the refractive fluid 218 disposed between themask 204 and the radiation source 208, it should be appreciated that therefractive fluid 218 may desirably be selected to have the third indexof refraction similar to the second refractive index. In particular, therefractive fluid 218 may be selected to have the third index ofrefraction substantially the same as the second index of refraction.Refraction of the radiation beams 210 at the refractive boundaries 211between the radiation source 208 and the mask material 206, and betweenthe mask material 206 and the radiation-sensitive material 203, maythereby be minimized.

To militate against potential heat transfer problems associated withhaving the radiation source 208 immersed in the refractive fluid 218,the radiation source 208 may be coupled with a lens 222. The lens 222has a thickness sufficient for the lens 222 to be at least partiallyimmersed in the refractive fluid 218 and for the bulk of the radiationsource 208 to remain dry above the reservoir 220. The lens 222 may haveparallel sides substantially normal to the radiation beams 210 and is atleast partially immersed in the reservoir 220 of the refractive fluid218. Illustratively, the lens 222 is formed from a substantiallyradiation transparent material, such as quartz glass. Other suitableradiation transparent materials may also be employed. The lens 222 mayhave a refractive index that is substantially the same as the thirdrefractive index of the refractive fluid 218. It should be appreciatedthat any refractive index of the lens 222 may be acceptable if theincident angle (α) is approximately zero. If the lens 222 were to beslightly rotated, for example, intentionally or by tolerance, it shouldbe further appreciated that there is a slight benefit from matchingindices of refraction. The lens 222 may thereby militate againstrefraction of the radiation beams 210 at the refractive boundary 211between the lens 222 and the refractive fluid 218. The lens 222 mayfurther be configured to allow the radiation beams 210 to pass throughthe refractive boundary 211 formed by the refractive fluid 218 and thelens 222 at an angle substantially normal relative to the boundary.

In a further embodiment shown in FIG. 4, the mask 204 has a plurality offacets 228 formed therein that provide the normalizing surfaces 224. Themask 204 may be formed in separate sections, for example, in the shapeof a Fresnel lens to provide the plurality of facets 228. The pluralityof facets 228 may be molded or cut into the surface of the mask 204. Therelative angles of the facets 228 may be the same or different acrossthe surface of the mask 204, and may be selected as desired. Inparticular, the facets 228 are configured to minimize the bending of theradiation beams 210 at the refractive boundary 211 between the radiationsource 208 and the mask material 206, and in particular between the airand the facets 228.

Referring to FIG. 5, the refractive fluid 218 may be disposed in theform of a layer between the mask 204 and the radiation-sensitivematerial 203. The refractive fluid 218 may be applied to theradiation-sensitive material 203 before placing the mask 204 on top ofthe radiation-sensitive material 203, for example. As a nonlimitingexample, where the system 200 includes the refractive fluid 218 disposedbetween the radiation-sensitive material 203 and the mask 204, it shouldbe appreciated that the refractive fluid 218 may be selected to have thethird refractive index between the first refractive index ofradiation-sensitive material 203 and the second refractive index of themask material 206. It should be further appreciated that the thirdrefractive index may desirably be substantially the same as the firstrefractive index. In particular embodiments, the refractive fluid 218may be selected to have the third refractive index greater than, or lessthan, each of the first and second refractive indices, as desired. Arefraction of the radiation beams 210 at the refractive boundary 211 ofthe radiation-sensitive material 203 and the mask material 206 maythereby be minimized, and a desired angle of the truss element 202thereby produced.

As shown in FIGS. 6-8, the system 200 having the normalizing surface 224may be used in conjunction with the refractive fluid 218 to minimizerefraction of the radiation beams 210 throughout the system 200, and tomaximize the critical angle (θ_(c)) at the refractive boundaries 211.With particular reference to FIG. 6, the system 200 may include theprism 226 disposed atop the mask 204 and the layer of refractive fluid218 disposed between the mask 204 and the radiation-sensitive material203. As further shown in FIG. 7, the system 200 may include the prism226, the radiation-sensitive material 203, and the mask 204 at leastpartially immersed in the reservoir 220 of the refractive fluid 218.Desirably, the refractive fluid 218 between the prism 226 and the mask204 militates against the presence of an air filled gap therebetween,which would otherwise provide the refractive boundary 211 with anundesirable critical angle (θ_(c)). As also shown in FIG. 8, the system200 may include the mask 204 with the plurality of facets 228 and thelayer of refractive fluid 218 disposed between the mask 204 and theradiation-sensitive material 203.

The present disclosure includes a method for fabricating theradiation-cured structure 201. The method first includes the step ofproviding the radiation-sensitive material 203 having a first refractiveindex, the mask 204 formed from the mask material 206 having the secondrefractive index, and the radiation source 208 configured to generatethe radiation beams 210 for at least one of initiating, polymerizing,crosslinking, and dissociating the radiation-sensitive material 203. Themask 204 has the plurality of radiation transparent apertures 212 and isplaced between the radiation-sensitive material 203 and the radiationsource 208.

The method of the present disclosure employs steps that militate againstdispersive refraction and total internal reflection of the radiationbeams 210 at the various refractive boundaries 211 within the system200. For example, the method includes at least one of the steps of a)disposing the at least one normalizing surface 224 between the radiationsource 208 and the mask 204, b) disposing the refractive fluid 218between the radiation source 208 and the mask 204, and c) disposing therefractive fluid 218 between the mask 204 and the radiation-sensitivematerial 203. The step of disposing the refractive fluid 218 between theradiation source 208 and the mask 204 may include the step of at leastpartially immersing at least one of the radiation source 208, the mask204, and the radiation-sensitive material 203 in the reservoir 220 ofthe refractive fluid 218. In another embodiment, the step of disposingthe refracting fluid 218 between the mask 204 and theradiation-sensitive material 203 may include the step of applying thelayer of the refractive fluid 218 to the radiation-sensitive material203 prior to placing the mask 204 adjacent thereto. In a furtherembodiment, the step of disposing the at least one normalizing surface224 between the radiation source 208 and the mask 204 may include thestep of placing the prism 226 atop the mask 204. Alternatively, the stepof disposing the at least one normalizing surface 224 between theradiation source 208 and the mask 204 may include the step of providingthe mask 204 with the surface having the plurality of facets 228. Inanother embodiment, the step of disposing the refractive fluid 218between the radiation source 208 and the mask 204 includes the furtherstep of disposing the refractive fluid 218 between the mask 204 and theprism 226. One of ordinary skill in the art should appreciate that thevarious steps described for militating against dispersive refraction andtotal internal reflection of the radiation beams 210 within the system200 may be employed individually, or in combination, within the scope ofthe present disclosure.

The radiation-sensitive material 203 is subsequently exposed to theplurality of radiation beams 210 through the radiation transparentapertures 212 in the mask 204. The radiation-cured structure 201 havingthe radiation-cured elements 202 is thereby formed. It should beappreciated that a variety of the radiation-cured elements 202 may beformed according to the present method, including truss elements,radiation-cured sheets, and solid radiation-cured polymer structures.

The refractive fluid 218 may be selected to have the desired thirdrefractive index, for example, to closely match the refractive indicesof adjacent media such as the radiation-sensitive material 203 and themask material 206. As nonlimiting examples, the radiation-sensitivematerial 203 and the refractive fluids 218 may be selected based on therespective refractive indices shown in Table 1 below.

TABLE 1 MEDIA INDEX OF REFRACTION AIR 1.000 WATER 1.333 ACETONE 1.360FUSED SILICA 1.458 FUSED QUARTZ 1.460 CARBON TET CL 1.461 PYREX GLASS1.470 SUGAR IN WATER, 80% 1.490 SUGAR IN WATER, 85% 1.503 LIQUIDPHOTOMONOMER 1.510 CROWN GLASS 1.520 SOLID PHOTOMONOMER 1.556 PET 1.575SU-8 PHOTORESIST 1.590

In certain embodiments, the refractive fluid 218 is selected to have thethird refractive index intermediate the first refractive index of theradiation curable material 203 and air. For example, where theradiation-sensitive material 203 is SU-8 photoresist (having an index ofrefraction of about 1.590) and the mask material 206 is fused quartz(having an index of refraction of about 1.460), an eighty percent (80%)sugar water solution (having an index of refraction of about 1.490) maybe selected for placement as the refractive fluid 218 between theradiation curable materials 203 and the mask material 206. The sugarwater solution displaces any air that may otherwise be disposed betweenthe radiation curable materials 203 and the mask material 206.

The refractive fluid 218 may be selected to have the third refractiveindex substantially the same as the first refractive index. For example,where the radiation-sensitive material 203 is a liquid photomonomer(having an index of refraction of 1.510), an eighty-five percent (85%)sugar water solution (having an index of refraction of about 1.503) maybe selected for placement adjacent the radiation-sensitive material 203.

The refractive fluid 218 may also be selected to have the thirdrefractive index between the refractive index of air and the secondrefractive index. For example, where the mask material 206 is crownglass (having an index of refraction of about 1.520), water (having anindex of refraction of about 1.333) may be selected for placementbetween the spaced apart radiation source 208 and the mask 204. Inparticular, the radiation source 208 may be at least partially immersedin the water to minimize refraction of the radiation beams 210 generatedby the radiation source 208.

The present method may further include the step of selecting the angleof the normalizing surface 224 to allow the radiation beams 210 tocontact the normalizing surface 224 at an angle substantiallyperpendicular to the normalizing surface 224. The method may alsoinclude the step of selecting the angle of the normalizing surface 224to allow the radiation beams 210 to contact the normalizing surface 224at an angle with respect to normal that is less than an angle theradiation beams 210 would otherwise contact absent the normalizingsurface 224. The refraction of the radiation beams 210 as they passthrough the normalizing surface 224, and the difference between theangle of incidence (α) and the refracted angle (θ) of theradiation-cured element 202, may thereby be minimized.

EXAMPLE

The following example is merely illustrative and does not in any waylimit the scope of the disclosure as described and claimed.

The radiation-cured structure 201 with at least one radiation-curedelement 202 having an angle of about seventy degrees (70°) relative tonormal to the plane of the refractive boundary 211 is fabricatedaccording to the present example. To militate against dispersive prismeffects which can appear at each refractive boundary 211, even onparallel faced bodies, the indices of refraction between different mediaare matched as closely as possible, and where a difference in index ofrefraction is unavoidable, the radiation beams 210 are caused to crossthe refractive boundary 211 at an angle about normal to the refractiveboundary 211.

The radiation-sensitive material 203 is SU-8 negative photoresist. TheSU-8 negative photoresist has a mask 204 formed from crown glass that isdisposed thereon. A layer of refractive fluid 218 consisting of a sugarwater solution is disposed between the SU-8 and the crown glass mask 204and prism 226. The radiation source 208 is spaced apart from the mask204. Three distinct refractive boundaries 211 are formed at theinterfaces of the various media, having different indices of refractionas shown in Table 2 below.

TABLE 2 INDEX OF MEDIA REFRACTION ANGLE WRT NORMAL SU-8 1.590 70.0 SUGARIN WATER, 85% 1.503 83.8 CROWN GLASS 1.520 79.4 AIR 1.000 0.0

As shown in Table 2 and in FIG. 9, a prism 226 having a normalizingsurface 214, for example, at an angle of about 10.6 degrees relative tothe refractive boundary 211 is employed. Radiation beams 210 travelingthrough the air from the radiation source 208 intersect the refractiveboundary 211 formed by the air and the crown glass at the angle ofincidence (α) of about 0° relative to normal. The refracted angle (θ) ofthe radiation beams 210 after crossing the refractive boundary 211 intothe crown glass remain at about 0° relative to normal to the refractiveboundary 211. A prism 226 having a normalizing surface 224, for example,at an angle of about 10.6 degrees relative to the refractive boundaries211 is employed to provide the angle of incidence (α) of about 73.8°relative to normal to the refractive boundary 211. As the radiationbeams 210 travel through the prism 226 to the mask 204, the refractedangle (θ) is about 79.4° relative to normal to the refractive boundary211 at the crown glass to sugar water interface.

The sugar water solution displaces any air that might otherwise bedisposed between the crown glass and the SU-8 negative photoresist.Since the indices of refraction of the crown glass and the sugar watersolution are similar, the refracted angle (θ) is about 83.8° relative tonormal to the refractive boundary 211 after the radiation beams 210cross into the sugar water solution. Upon the crossing of the radiationbeams 210 into the SU-8 negative photoresist, it is expected that therefracted angle (θ), and thereby the angle of the radiation-curedelement 202, is about 70° relative to normal to the refractive boundary211. The formation of large-angled radiation beams 210 and relatedradiation-cured elements 202 having angles greater than about 45°, andparticularly greater than about 60°, relative to normal to therefractive boundary 211 is thereby facilitated.

It is surprisingly found that, in addition to the facilitating thefabrication of the radiation-cured structures 201 with large-angledfeatures, the approach of the present disclosure facilitates theproduction of structures at angles that were marginally feasible withpreviously known methodology. It should be appreciated that, because thecritical angle (θ_(c)) is derived from the indices of refraction for theinitial and final optical media such as air and the radiation-sensitivematerial 203 or the mask material 206, the critical angle (θ_(c))limitation can be militated against by substituting the refractive fluid218 for the air. The system 200 and method is designed such that theindices of refraction between the refractive boundaries 211, such asbetween the radiation-sensitive material 203, and the mask material 206,may be minimized. The system 200 is also designed such that theradiation beams 210 pass through the refractive boundaries 211 of thesystem 200 at angles desirably close to normal, thereby furtherminimizing dispersive refraction and sensitivity to changes in theindices (such as due to changes in temperature, pressure, and RH of theair, for example), the radiation beams 210, and the related criticalangle (θ_(c)). TIR conditions are thereby mitigated.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes may be made without departingfrom the scope of the disclosure, which is further described in thefollowing appended claims.

1. A method for fabricating a radiation-cured structure, the methodcomprising the steps of: providing a radiation-sensitive material havinga first refractive index, a mask having a plurality of substantiallyradiation transparent apertures, the mask formed from a material havinga second refractive index, and a radiation source configured to generateradiation beams for at least one of initiating, polymerizing andcrosslinking the radiation-sensitive material; placing the mask betweenthe radiation-sensitive material and the radiation source, the maskabutting the radiation-sensitive material; disposing a refractive fluidhaving a third refractive index between the radiation source and themask, wherein the refractive fluid is selected to have one of the thirdrefractive index intermediate the first refractive index and the secondrefractive index, the third refractive index substantially the same asthe first refractive index, and the third refractive index between therefractive index of air and the second refractive index; and exposingthe radiation-sensitive materials to a plurality of radiation beamsthrough the radiation transparent apertures in the mask, whereby theradiation-cured structure is formed.
 2. The method of claim 1, whereinthe step of disposing the refractive fluid between the radiation sourceand the mask includes the step of at least partially immersing theradiation source, the mask, and the radiation-sensitive material in therefractive fluid.
 3. A method for fabricating a radiation-curedstructure, the method comprising the steps of: providing aradiation-sensitive material having a first refractive index, a maskhaving a plurality of substantially radiation transparent apertures, themask formed from a material having a second refractive index, and aradiation source configured to generate radiation beams for at least oneof initiating, polymerizing and crosslinking the radiation-sensitivematerial, a prism integral with the mask and not having air disposedtherebetween; placing the mask between the radiation-sensitive materialand the radiation source, an at least one normalizing surface disposedbetween the radiation source and the radiation-sensitive material, theat least one normalizing surface provided by the prism of the mask, andexposing the radiation-sensitive materials to a plurality of radiationbeams through the radiation transparent apertures in the mask, wherebythe radiation-cured structure is formed.
 4. The method of claim 3,further comprising the step of selecting the angle of the normalizingsurface to allow the radiation beams to contact the normalizing surfaceat about normal to the normalizing surface, thereby minimizing arefraction of the radiation beams as they pass through the normalizingsurface.
 5. A method for fabricating a radiation-cured structure, themethod comprising the steps of: providing a radiation-sensitive materialhaving a first refractive index, a mask having a plurality ofsubstantially radiation transparent apertures, the mask formed from amaterial having a second refractive index, and a radiation sourceconfigured to generate radiation beams for at least one of initiating,polymerizing and crosslinking the radiation-sensitive material; placingthe mask between the radiation-sensitive material and the radiationsource, the mask having a faceted surface with a plurality of facetsproviding a plurality of normalizing surfaces, the plurality of facetsminimizing a bending of the radiation beams at a refractive boundarybetween the radiation source and the mask; and exposing theradiation-sensitive materials to a plurality of radiation beams throughthe radiation-transparent apertures in the mask, whereby theradiation-cured structure is formed.
 6. The method of claim 5, furthercomprising the step of selecting the angle of the normalizing surfacesto allow the radiation beams to contact the normalizing surfaces atabout normal to the normalizing surfaces, thereby minimizing arefraction of the radiation beams as they pass through the normalizingsurfaces.